Yes, I agree. I already modified the resource_prop_reader method to accept classes ( you can see two examples in the application class). However, this is clumsy to do with the account_store property since it can be either a group or a directory. 

One of the ideas would be to see if href exists, and if it does, to take the class from it (in a REST URL, it's always the second from the back if you split it by '/'). Something such as:

  class Association
    def get_resource_property(key, clazz)
      value = get_property key

      if value.is_a? Hash
        href = get_href_from_hash value
      end
      
      if href
        clazz = href.split('/')[-2].camelize.constantize
        data_store.instantiate clazz, value
      end
    end 
  end

However, this href.split('/')[-2] looks dirty to me, although all of the tests did pass it.